Reactions with direct transfer of hydrogen between nicotinamide coenzyme and substrate

This is a small monomeric enzyme (molecular weight around 20000) that catalyses reduction of a double bond between carbon and nitrogen, converting 7,8-dihydrofolate into 5,6,7,8-tetrahydrofolate (Fig. 14). Hydride is transferred from the 4-pro-R position of NADPH to C-6, N-5 acquiring a proton [64-66]. 

Crystallographic studies at 2.5 A resolution have been carried out on the enzyme from E.coli [67] (Fig. 15) and Lactobacillus casei [68,69] (Fig. 16). Residue numbers in what follows refer to the L. casei enzyme sequence [70]. The binding sites for the substrate and coenzyme are not located in separate domains, but are composed of overlapping portions involving mainly the N-terminal two-thirds of the polypeptide 

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